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Re: Lumber Moment Frame Connection

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Okay, I usually don't use moment frames in wood, nor do I count knee braces in timber structures, and the easier answer is to embed the poles. However, I'm going to take Jim's position here for the sake of argument.

We all know how wood reacts with fasteners. We all know statics. In a purely academic sense, we can determine the loads, forces, and reactions on just about any simple structure by hand. Presuming that you use an R=1, a phenomenally conservative value (as I will expand later), there is no reason that the combination of steel design - including buckling stability - and wood fastener design - using the 6 interaction equations - can't be used to make a moment frame of defined capacity. If this were not the case, then shear connections without bearing would be forbidden! That 2x2 ledger supporting a joist would never last, as the forces over time along with the shrinking of the wood would allow the nails to deflect and the system to collapse. You couldn't bolt a ledger into a band - the same thing would happen, but even worse since the fastener isn't a displacement type. The band would flop around like a wet noodle causing anyone walking on the adjacent surface to fear for their lives. And hurricane ties and seismic hold-down anchors - wow, talk about a recipe for disaster.

As for the performance in a seismic event, what could be better than a moment connection in a non-linear material with mechanism that builds in a significant amount of hysteresis in the dynamic performance. Owners pay millions of dollars to implement such systems using dampers and plastic hinge connections. Would you give a lower R value to this type of system than you give to URM - possibly the worst conceivable lateral force resisting system for seismic loads?

I say there's no reason that a properly designed and installed steel-braced moment frame system shouldn't be considered as a legitimate structural element. And don't bother trying to bring up the red herring of cross-grain bending - cross-grain bending occurs in every connection which uses more than one fastener.

Jordan


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